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MECHANICAL SEPARATION                              287
Separation by Weighting.—This term should probably be limited to open tank operations where the agent of separation is a finely divided powder which on settling through the liquid in the tank carries down by simple attraction and entraining suspended solids. Perhaps as simple an example of this action as is to be found is described in U. S. Patent #251938 and others bearing on separating flocculent cotton as cellulose from acids in the production of celluloid and similar substances, the weighting down agent being powdered barium sulphate. This is introduced at the top of the open tank and in settling down through the acid carries with it the suspended particles of cotton. The other pole of separatory processes coming under this head would be chemical precipitation.
The action of some of the weighting agencies are purely adsorptive. Silica gel. is used this way on some problems to which reference will be made later. Silica gel is a reversible adsorbent. It can be used as a filter as is bone char, one way of using fuller's earth, etc., or it can be used as a weighting agency in an open tank. Adsorption alone can produce important changes of state. In the preparation of "gray powder" mercury is triturated with chalk and becomes a fine gray powder from which is made mercurial ointment. The trituration is assisted by the holding apart action of the chalk. The small mercury particles finally resulting are prevented from coalescing by adsorbed gas transferred to them from the gas envelopes of the chalk particles.
Sewage.—With the great advance of the bacterial methods of disposing of sewage the agencies described below are fast becoming antiquated. In liming sewage (Etienne's patent, 1802) the lime first forms a carbonate and this in settling carries down suspended solids and organic matter which forms compounds of uncertain character. Magnesium chloride, aluminum sulphate and phosphate, barite and other salts have been proposed and used. In addition to the reagents clay, charcoal and other weighting and absorbent bodies have been proposed for use in connection with them. Aluminum salts form certain insoluble compounds with organic matter, the chemical and other actions being similar to those which take place in the manufacture of the lake colors. In the preparation of these colors to an infusion of organic coloring matter such as madder, cochineal, logwood or quercitron is added common alum which produces a precipitate of aluminum hydrate which in settling carries down the coloring matter forming the lake.
Adsorbents.—From the point of view of mechanical separation silica gel can be used in three principal ways. First as a filter (removal of moisture in vacuum-refrigerating plant, refining of paraffin, removal of vapors of alcohol, ether, benzol, acetone etc.) Second as a dust brought into contact with the vapor to be removed with recovery by cloth filtration (vapor from the blast of blast furnaces etc.) with activation by heating. Third by mixing the silica gel with a liquid to be refined allowing the silica gel with its impurities to settle, displacing the adsorbed impurities by water, filtering and activating by heat. This method of using the adsorbent is proposed for refining gasolene and kerosene and decolorizing operations. Counter current decantation fits in very nicely with this mode of employing the adsorbent.
That silica and silicates possess strong absorptive power has been known for a long time as witness the use of diatomaceous earth, fuller's earth, clay, talc, etc. for adsorptive, absorptive and decolorizing processes. Silica gel is prepared from